Stabilized jet combustion fuels



United States Patent 3,155,463 STABILIZED 3E1 C(BMBUSJHGN FUELS Harry J. Andress, .lr., Pitrnan, and Paul Y. C. Gee, Woodhury, NJBZ, assignors to Socony Mobil Oil Company, Inc., a corporation of New York No Drawing. Filed MM. 8, 1962, Ser. No. 178,251 16 Claims. (Cl. 44-62) This invention relates to jet combustion fuels that are stable at relatively high temperatures. It is more particularly concerned with jet combustion fuels adapted for use in high temperature jet engines and with novel additive composition therefor.

As is well known to those familiar with the art, aviation turbine engines, or jet engines, are operated at extremely high temperatures, particularly in the case of supersonic jet aircraft engines. In order to remove some of the heat and also preheat the incoming fuel, the fuel is subjected to indirect heat exchange with the combustion chamber. Then, when passing through the injection nozzles, the incoming fuel is further subjected to high temperature conditions. Many jet fuels have been found to be relatively unstable when subjected to high temperature. ecomposition products are formed which tend to foul the heat exchange tubes and to cause plugging of the injection nozzles. As will readily be appreciated, the use of such fuels results in shortened operational life of the engine and can be a source of hazard in the operation of the jet aircraft. Accordingly, a means of stabilizing such fuels against degradation is highly desirable.

It has now been found that thermally unstable jet combustion fuels can be stabilized against degradation simply and economically. It has been discovered that the addition of a small amount of a mixture comprised predominantly of an alkylated catechol in combination with certain polymeric substances and the disalicylaldimine of propylene or ethylenediamine will stabilize jet combustion fuels against thermal degradation, thereby minimizing the fouling of heat exchange tubes and the plugging of the nozzles.

Accordingly, it is an object of this invention to provide stable jet combustion fuels. Another object is to provide a means for stabilizing jet combustion fuels against thermal degradation. A further object is to provide jet combustion fuels having a greatly reduced tendency to foul heat exchange tubes and to plug injection nozzles. A speciiic object is to provide jet combustion fuels containing an additive composition of an alkylated catechol, certain metal deactivating compounds, and certain partially esterified copolymeric substances. Other object and advantages of this invention will become apparent to those skilled in the art from the following detailed description.

The present invention is based on the discovery that petroleum hydrocarbon jet fuel compositions that are not sufficiently inhibited against thermal degradation by an alkylated catechol or by a mixture of (1) certain polymeric substances and (2) certain disalicylaldimines are markedly improved against thermal degradation by use of an additive consisting essentially of such an alkylated catechol in combination with such a mixture. Thus, and although for certain fuels the individual use of either of the components of such a combination may provide adequate stabilization against thermal degradation for certain requirements, other fuels oftentimes do not adequately respond to individual use of such components. More specifically, with certain petroleum hydrocarbon distillate jet fuel compositions, the use of an additive (A) consisting essentially ..of (l) a disalicylaldimine of propylene diamine or ethylene diamine and (2) certain reaction products of a partially esterified l-olefin maleic anhydride polymer reacted with an alkylpolyamine and salicylaldehyde adequately inhibits the fuel against filter plugging or preheater deposits,

3,155,463 Patented Nov. 3, 1 964 or both; and some fuels also responds in similar manner to alkylated catechols. However, for fuels that do not respond to either additive (A) or the alkylated catechol, used individually, to the extent of adequately inhibiting against both filter plugging and formation of preheater deposits, it has been found that use in combination of additive A and an alkylated catechol provide markedly improved inhibition against both filter plugging and formation of preheater deposits.

Thus, the present invention is based on the discovery that a monoor poly-alkylated catechol in combination with a relatively minor proportional amount of a mixture of (l) a disalicylaldimine of propyleneor ethylenediamine and (2) certain products of a partially esterified 1-olefin-maleic anhydride polymer reacted with an alkylene polyamine and salicylaldehyde will stabilize a petroleum hydrocarbon jet fuel against thermal degradation, including inhibition against both filter plugging and formation of preheater deposits and, especially so, will so stabilize such fuels, particularly those derived from naphthenic crudes, that do not sufficiently respond in both of those characteristics by individual use of the components of the combination additive embodied herein. More specifically, a combination additive as embodied herein, used in amounts of from about 0.001 to about 1.0% based on the weight of the jet combustion fuel, has been found to provide jet combustion fuels that not only are iughly' satisfactory in being substantially non-emulsive but, as described more fully hereinafter, the fuel composition is inhibited against both filter plugging and formation of preheater tube deposits and gums.

The novel combination additive embodied herein is comprised essentially of an allrylated catechol in predominant amount and, for example, from about to weight percent, with the balance of the combination additive consisting essentially of from about 5 to about 25% of the mixture of a disalicylaldimine and a polymeric substance as aforedefined. In such mixture, the ratio of the disalicylaldimine to the polymeric substance is, preferably, in substantially equal parts by weight but, if desired, the ratio ray be varied to suit particular requirements and, for example, the ratio of the disalicylaldimine to the polymeric substance may in general be varied from about 25 to about 75 parts of the disalicylaldimine to 75 to 25 parts of the polymeric substance.

In reference to the alkylated catechols, i.e., the predominant component of the combination additive embodied herein, they can be prepared by conventional procedures and, as an embodiment thereof, by reacting catechol with an oleiin (e.g., propylene tetramer) in the presence of an allrylation catalyst (e.g., boron trifiuoride ethyletherate) at from about 50 to about C. to alkylate the catechol. For practice of this invention, the use of alkylated catechols having at least one relatively long alkyl chain, e.g., from C to C carbon atoms, is preferred and, suitable examples thereof include butyl, octyl, decyl, dodecyl, and hexadecyl.

One of the other components of the combination additive embodied for use herein is the reaction product obtained by (1) esterifying a (a) 1:1 molar copolymer of a l-olefin, having between about 2 carbon atoms and about 20 carbon atoms per molecule, and maleic acid anhydride with (:5) between about 1.5 moles and about 1.75 moles, per mole of said copolymer, of a primary or secondary aliphatic alcohol containing between about 4 carbon atoms and about 20 carbon atoms per molecule to produce a mixed monoand di-esterproduct; and (2) reacting said ester product with between about 0.25 mole and about 0.50 mole of a polyamine of the formula H N(RNH) H, wherein R is ethylene or propylene and n is l 'or 2, and between about 0.25 mole and amine and salicylaldehyde.

3 about 0.50.1nol of salicylaldehyde per mole of said ester products.

In preparation of such a mixed monoand di-ester product (for subsequent reaction with the polyamine and salicylaldehyde) the heteropolymer (for esterification) may be produced by copolymerizing cquimolar amounts of a l-olefin and maleic acid anhydride. The l-olefin reactant should contain between about 2 carbon atoms and about 20 carbon atoms per molecule. The reactants are heated together, either in bulk, or in the pres ence of a suitable solvent, such as benzene, toluene, xylene, dioxane, or carbon tetrachloride, at temperatures varying between about 75 C. and about 175 C. Preferably, the copolymerization is carried out in the presence of a peroxide catalyst, such as benzoyl peroxide. The peroxide may be used in an amount of between about one percent and about 5 percent, by weight of the reactants, and the time required to complete the copolymerization generally varies between about one hour and about hours.

The mixed monoand di-aliphatic ester is made, as aforesaid, by esterifying the heteropolymer with a primary or secondary aliphatic alcohol containing between about 4 carbon atoms and about 20 carbon atoms per molecule and, of these, branched-chain alkyl alcohols are particularely preferred. Non-limiting examples of the esterifying alcohols are butanol, 2-methyl-propanol, 2,2-dimethylpropanol, amyl alcohol, isoamyl alcohol, hexanol, hexenol, 3-rnethyl-pentanol, 2-ethylhexanol, isodecanol, decanol, dodecanol, iso-tridecanol, hexadecanol, hexadecenol, octadecanol, octadecenol, and eicosanol. It is to be noted that the esters utilizable herein are mixed monoand di-aliphatic esters. Accordingly, the esterification is carried out with ues of between about 1.5 moles and about 1.75 moles of the alcohol per mole of the olefinmaleic anhydride copolymer.

As will readily be appreciated by those familiar with the art, the aforesaid ester can be made by any of the known methods for preparing esters of carboxylic acids. For example, the mixed esters of l-olefin-maleic anhydride copolymers can be prepared by heating at 100 200 C., from 1 to 10 hours, one mole of l-olefin-maleic anhydride copolymer with 1.5-1.75 moles of alcohol with the elimination of 0.5-0.75 mole of water. The esterification is suitably carried out in the presence of a catalyst, such as p-toluene sulfonic acid monohydrate or sulfuric acid. The amount of p-toluene sulfonic acid monohydrate or sulfuric acid used is from 0.1 percent to 5 percent by weight of the reactants. Likewise, there can be used various techniques of esterification, such as azeotropic distillation, or removal of water by the use of applied vacuum.

The mixed monoand di-aliphatic ester component is then further reacted with, per mole of mixed ester, between about 0.25 mole and about 0.5 mole of a polyamine reactant as hereinafter defined and between about 0.25 mole and about 0.5 mole of salicylaldehyde. The polyamine reactant has the formula H N (-RNH),,H, wherein R is ethylene or propylene and n is 1 or 2. Examples of the polyamine reactant are ethylenediamine, propylenediamine, diethylenetriamine and dipropylenetriamine. The reaction between the copolymer ester, polyamine reactant, and salicylaldehyde is a condensation reaction accompanied by the formation of water of reac tion. The reaction is suitably carried out at temperatures varying between about 90 C. and about 175 C. and for a period of time varying between about one hour and about 5 hours, or until water of condensation ceases to form. As was described in the case of the esterification step, various techniques of esterification (or condensation in general) are equally applicable to the reaction with the Thus, for example, 'Water can be removed by azeotropic distillation. Following both the esterification step and the step of reacting with amine and salicylaldehyde, it is desirable to water-wash to ensure removal of any water-soluble matter that may remain following each step.

The metal deactivator component of the additive compositions of this invention are aldehyde-amine condensation products of the type of arylidene amines or Schiifs bases having the formula:

wherein A represents a benzene ring, the OH radical being attached directly to a ring carbon atom ortho to the CH=N- group and R represents an aliphatic radical having the two N atoms attached directly to diiferent carbon atoms of the same open chain of R. These materials have been fully described in US. Letters Patent Nos. 2,181,121 and 2,284,267. Generally, they are produced by reacting two moles of salicylaldehyde with one mole of a diamine such as ethylene diamine or propylene diamine with the elimination of two moles of water condensation. The preferred metal deactivator is the N,N

disalicylaldimine of propylene diamine having the structure:

H 0 OH GHNCH,(I3HNCH The hydrocarbon jet fuels that are improved in accordance with this invention are hydrocarbon fractions having an initial boiling point of at least about 100 F. and an end boiling point as high as about 750 F. These fuels can be made up of straight-run dis illate fractions, catalytically or thermally cracked (including hydrocracked) distillate fractions, or mixtures of straight-run fuel oil, naphtha, etc. with cracked distillate stocks, alkylate, and the like. The principal properties that characterize the jet fuels is their boiling range. Each fuel will have a boiling range which falls within the aforespecified range. Specifications that define typical specific fuels are MIL-F-56l6, MIL-J-5624D, MIL-F-25656, MIL-F-2524A, MIL-F-25576A, MIL-F-25558B, and MIL-J-5l6lE.

The amount of additive composition, i.e., the combination aforedescribed, that is added to the jet combustion fuels will vary between about 0.001 percent and about one percent, by weight of the fuel, and preferably between about 0.01 percent and about 0.02 perwnt. In terms of weight per unit volume of fuel, the concentration of additive composition will vary between about 5.0 pounds per thousand barrels of fuel and about 2000 pounds per thousand barrels of fuel. Preferably, the

concentration will vary between about 25 and about 50 pounds per thousand barrels of fuel.

The following examples are for the purpose of illustrating preparation of additive compositions of this in- EXAMPLE I Preparation of Alkylated Catechols A mixture of 88 grams (0.8 mol) catechol, 269 grams (1.6 *mols) propylene tetramer and 15 grams boron trifluoride ethyl etherate was stirred at 100 C. for six hours. a The reaction mixture was then water washed until the'washings were neutral and, by topping under reduced-pressure, didodecyl catechol was obtained.

' EXAMPLE n p a A mixture of grams (0.5 mol) of l -dodecene, 49

grams (0.5 mol) of maleic anhydride, 2.66 grams (2%) of benzoyl peroxide, and 25 cc. of xylene was gradually heated with stirring to 100 C. The heat was then turned off as the reaction was exothermic with a temperature rise to 155 C. The reaction mixture was then stirred for three hours at 155 C. to complete copolymerization. The resulting copolymer was diluted with 250 cc. xylene and cooled to room temperature, following which 175 grams (0.875 mol) of isotridecyl alcohol and 6.16 grams (2% of p-ttoluene sulfonic acid monohydrate were added. The mixture was then refluxed for about 4 hours at 150 C. and for about 2 hours at 175 C. About 7 cc. of water (theory=6.75 cc.) was given off. The resulting ester mixture was diluted with benzene and water washed until the washings were neutral to litmus paper. To the diluted partial ester of the copolymer was added 15.25 grams (0.25 mol) of salicylaldehyde and 7.5 grams (0.25 mol) of ethylene diamine. The mixture was refluxed at 95 C. for two hours and at 150 C. for two hours with a yield of 5 cc. of water (theory=4.5 cc), and then topped under reduced pressure to produce the condensation product of the salicylaldehyde, ethylene diamine and the esterified copolymer.

EXAMPLE III A composition, comprising an additive mixture embodied for use herein, was prepared by blending fifteen parts by weight of the didodecyl catechol of Example I, one part by weight of the reaction product of Example II (the reaction product of partially tridecyl esterified dodecene-l-maleic anhydride co-polymer with ethylene diamine and salicylaldehyde), and one part of the disalicylaldimine of propylene diamine.

Each of the products of the foregoing examples was subjected to test under the conditions of a test method used for determining the thermal stability characteristics of aviation turbine fuels. The method was developed by the Coordinating Research Council as published in CRC Report Investigation of Thermal Stability of Aviation Turbine Fuels with CFR Fuel Coker (CRC Project CFA254), July 1957. The method is set forth in detail in Appendix XV of the ASTM Standards on Petroleum Products and Lubricants November 1957, cornmencing at page 1059. This method provides a means for measuring the high temperature stability of aviation turbine fuels, using an apparatus known as the CFR Fuel Coker, which subjects the test fuel to temperatures and conditions similar to those occurring in some aviation turbine engines. Fuel is pumped, at a rate of about 6 pounds per hour, through a preheater section which simulates the hot fuel line sections of the engine as typified by an engine fuel-oil cooler. It then passes through a heated filter section which represents the nozzle area or small fuel passages of the hot section of engine where fuel degradation products may become trapped. A precision sintered stainless steel filter in the heated filter section traps fuel degradation products formed during the test. The extent of the build-up is noted as an increased pressure drop across the test filter and, in combination with the deposit condition of the preheater, is used as an assessment of the fuels high-temperature stability. In the testing described herein, the filter temperature was 500 F and the preheater tube temperature was 400 F. In each run the test was continued until there was a pressure drop of 25 inches of mercury across the filter or until a time of 300 minutes had elapsed, whichever occurred first. In order to be satisfactory in the test, .a fuel should show little or no pressure drop across the filter at the end of the 300 minutes. The preheater deposits in the tests are evaluated according to a code rating varying from 0 to 4 wherein Code 0 means no visible deposits; Code 1, haze or dulling, no color; Code 2, barely visible discolorization; Code 3, light tau; and Code 4, heavier than Code 3. It will be readily appreciated that a rating lower than Code 3 is desirable for an effective stable aviation turbine fuel.

The base hydrocarbon jet combustion fuel used here-in was a straights-1m petroleum fraction boiling between about 325 F. and about 525 F. A portion of this base fuel, uninhibited was subjected to the Fuel Coker Test. Then, other portions of the base fuel was blended with the reaction products of the foregoing examples, and each blend so obtained was subjected to the Fuel Coker As is apparent from the foregoing data, the uninhibited jet fuel (Run 1) showed considerable pressure drop across the filter and had an unsatisfactory rating (Code 4) with respect to preheater deposits. Although some improvement occurred with respect to pressure drop across the filter by the addition of 50 lbs./ 1000 barrels of didodecyl catechol (Run 2), the improvement was not substantial and the fuel composition still provided a Code 4 rating. Similarly, and although addition of the metal deactivator (Run 3) provided some improvement against filter plugging, the fuel composition still had a code rating of 4, as did the fuel containing the reaction product of Example 2. The marked improvement that results from practice of this invention is evident from the data set forth for Run 5. As shown the fuel composition containing the combination additive embodied herein passed both requirements of the test, i.e., Code 0 and no pressure drop across the filter after 300 minutes.

Although the present invention has been described with preferred embodiments, it is to be understood that modifica-tions and variations may be resorted to, without departing from the spirit and scope of this invention, as those skilled in the ant will readily understand. Such variations and modifications are considered to be Within the purview and scope of the appended claims.

What is claimed is:

1. An additive composition, adapted for stabilizing petroleum hydrocarbon distillate jet combustion fuels against thermal degradation, consisting essentially of (1) an alkylated catechol containing at least one alkyl group of from about four to about sixteen carbon atoms in an amount of from about 75 to about by weight of said composition and (2) from about 2.5 to about 5% H by weight of said composition of a mixture of (a) a disalicylaldimine of a diamine from the group consisting of ethylene diam-inc and propylene diarnine and (b) a reaction product obtained by esterifying a 1:1 molar copolymer of a l-olefinic hydrocarbon, having between about 2 carbon atoms and about 20 carbon atoms per molecule, and maleic acid anhydride with between about 1.5 moles and about 1.75 moles, per mole of said copolymer, of an alcohol from the group consisting of primary and secondary aliphatic hydrocarbon monoalcohols containing between about 4 carbon atoms and about 20 carbon atoms per molecule to produce a mixed monoand di-ester product; and reacting said ester product with between about 0.25 mole and about 0.50 mole of a polyamine of the formula H N(R-NH) H, wherein R is selected from the group consisting of ethylene and propylene and n varies between 1 and 2 and between about 0.25 mole and about 0.50 mole salicylaldehyde, per mole of said ester product, said additive composition being further characterized in that it contains said disalicylal dimine (a) and said reaction product (5) in a ratio by 7 weight of from about 25 to about 75 parts of said disalicylaldimine to from about 75 to about 25 parts of said reaction product.

2. A composition, as defined in claim 1, wherein the disalicylaldimine (a) and said reaction product (b) are present in substantially equal parts by weight.

3. A composition, as defined in claim 1, wherein the alkylated cateohol is didodecylcatechol.

4. A composition, as defined in claim 1, wherein the disalicylaldimine is the disalicylaldimine of propylene d-iamine.

5. A composition, as defined in claim 1, wherein the alkylated oatecho-l is didodecylcatechol, the disalicylaldimine i s the disalicylaldimineof propylene diamine, and

the reaction product (11) is prepared by use of l-dod ece ne as the l-olefin, isotridecyl alcohol as the alcohol for esterifying the copolymer, and ethylene diamine as the polyamine.

6. A composition, as defined in claim 5, that consists essentially in Weight ratio, of about 15 parts of the alkylated catechol, about one part of the disalicylaldimine and about one part of the reaction product.

7. A petroleum hydrocarbon distillate jet combustion fuel containing a small amount, suiiicient to mhibit said fuel against thermal degradation, of the additive composition of claim 1.

8. A fuel, as defined in claim 7, that contains the additive composition of claim 1 in an amount of from about 0.001 to about 1.0% based on the weight of the fuel.

9. A liquid petroleum hydrocarbon distillate fuel having an initial boiling point of at least 100 F. and an. end boiling point as high as about 750 F. containing from about 0.001 to about 1.0% by weight of said fuel, and suiiicient to inhibit said fuel against thermal degradation, of the additive composition of claim 1.

10. A liquid petroleum hydrocarbon distillate fuel having an initial boiling point of at least 100 F. and an end boiling point as high as about 750 F. containing from about 0.001 to about 1.0% by Weight of said fuel, and suflicient to inhibit said fuel against thenrnal degradation, of the additive composition of claim 2.

11. A liquid petroleum hydrocarbon distillate fuel having an initial boiling point of at least 100 F. and an end 8 boiling point as high as about 750 F. containing from about 0.001 to about 1.0% by weight of said fuel, and sufiicient to inhibit said fuel against thermal degradation, of the additive composition of claim 3.

12. A liquid petroleum hydrocarbon distillate fuel having an initial boiling point of at least F. and an end boiling point as high as about 750 F. containing from about 0.001 to about 1.0% by weight of said fuel, and .sufiicient to inhibit said fuel against thermal degrada tion, of the additive composition of claim 4.

13. A liquid petroleum hydrocarbon distillate fuel having an initial boiling point of at least 100 F. and an end boiling point as high as about 750 F. containing from about 0.001 to about 1.0 by weight of said fuel, 1 W

and sufiicient to inhibit said fuel against thermal degradation, of the additive composition of claim 5.

14. A liquid petroleum hydrocarbon distillate fuel having an initial boiling point of at least 100 F. and an end boiling point as high as about 750 F. containing from about 0.001 to about 1.0% by Weight of said fuel, and suflicient to inhibit said fuel against thermal degradation, of the additive composition of claim 6.

15. A petroleumv hydrocarbon distillate jet combustion fuel, having an initial boiling point of at least 100 F. and an end boiling point as high as about 750 F., containing a small amount, sufiicient to inhibit said fuel against thermal degradation, of the additive composition of claim 1.

16. A liquid petroleum hydrocarbon distillate fuel having an initial boiling point of at least 100 F. and an end boiling point as high as about 750 E, containing a small amount, sufiicient to inhibit said fuel against thremal degradation, of the additive composition of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,712,497 Fox et al. July 5, 1955 2,956,012 Mosier et al. Oct. 11, 1960 2,977,334 Zopf et al. Mar. 28, 1961 3,031,278 Buchmann et al. Apr. 24, 1962 

1. AN ADDITIVE COMPOSITION, ADAPTED FRO STABILIZING PETROLEUM HYDROCARBON DISTILLATE JET COMBUSTION FUELS AGAINST THERMAL DEGRADATION, CONSISTING ESSENTIALLY OF (1) AN ALKYLATED CATECHOL CONTAINING AT LEAST ONE ALKYL GROUP OF FROM ABOUT FOUR TO ABOUT SIXTEEN CARBON ATOMS IN AN AMOUNT OF FROM ABOUT 75 TO ABOUT 95% BY WEIGHT OF SAID COMPOSITION AND (2) FROM ABOUT 25 TO ABOUT 5% BY WEIGHT OF SAID COMPOSITION OF A MIXTURE (A) A DISALICYLLDIMINE OF A DIAMINE FROM THE GROUP CONSISTING OF ETHYLENE DIAMINE AND PROPYLENE DIAMINE AND (B) A REACTION PRODUCT OBTAINED BY ESTERIFYING A 1:1 MOLAR COPOLYMER OF A 1-OLEFINIC HYDROCARBON, HAVING BETWEEN ABOUT 2 CARBON ATOMS AND ABOUT 20 CARBON ATOMS PER MOLECULE, AND MALEIC ACID ANHYDRIDE WITH BETWEEN ABOUT 1.5 MOLES AND ABOUT 1.75 MOLES, PER MOLE OF SAID COPOLYMER, OF AN ALCOHOL FROM THE GROUP CONSISTING OF PRIMARY AND SECONDARY ALIPHATIC HYDROCARBON MONOALCOHOLS CONTAINING BETWEEN ABOUT 4 CARBON ATOMS AND ABOUT 20 CARBON ATOMS PER MOLECULE TO PRODUCE A MIXED MONOAND DI-ESTER PRODUCT; AND REACTING SAID ESTER PRODUCT WITH BETWEEN ABOUT 0.25 MOLE AND ABOUT 0.50 MOLE OF A POLYAMINE OF THE FORMULA H2N(-R-NH)N-H, WHEREIN R IS SELECTED FROM THE GROUP CONSISTING OF ETHYLENE AND PROPYLENE AND N VARIES BETWEEN 1 AN 2 AND BETWEEN ABOUT 0.25 MOLE AND ABOUT 0.50 MOLE SALICYLALDEHYDE, PER MOLE OF SAID ESTER PRODUCT, SAID ADDITIVE COMPOSITION BEING FURTHER CHARACTERIZED IN THAT IT CONTAINS SAID DISALICYLALDIMINE (A) AND SAID REACTION PRODUCT (B) IN RATIO BY WEIGHT OF FROM ABOUT 25 TO ABOUT 75 PARTS OF SAID DISALICYLALDIMINE TO FROM ABOUT 75 TO ABOUT 25 PARTS OF SAID REACTION PRODUCT. 